Recently, ones utilizing a digital system are noted in the technology of image formation by electrophotography. For example, a dot image formed by small dots on 1200 dpi level, in which dpi is the number of dot per inch, can be finely reproduced by employing a toner having a small diameter of several micrometers order.
It has been made possible to form a beautiful full color toner image by the use of color toners of yellow, magenta, cyan and black, which is no way inferior to a silver salt photographic image. There is a great need for reproducing a high precision digital image with high fidelity. A technique is applied as one of the countermeasures corresponding to the requirement, in which such the need is satisfied by controlling the physical properties of the toner to satisfy.
Among the conventional art, there is an art in which an attention is drawn to controlling water content of toner, for example, a method to form an image by employing toner whose saturated water content at circumstances of 30 C and 80% RH was adjusted to 0.1 to 2.0% by weight. (For example Patent Documents No. 1 and No. 2)
A toner having saturated water content of 0.1 to 2.0% by weight is employed in the above-mentioned art. There is a problem that an image defect of white spots called toner blister is generated by bubbles of vapor generated from toner layer when the image is formed by superposing four colored toners having high water content.
Alternatively when the four color toners all of them having low water content are employed, such other problems are caused that rising characteristics of toner charge is inferior, image density gradually becomes lower when image is formed continuously, toner content arises in excess, and toner stain within the apparatus is apt to caused.
It is preferred to make size of four color image forming unit, in particular, developing units same for the purpose of making the image forming an apparatus compact, or reducing production cost.
There is an image forming mode in which consumption of four color toners is different from each other as well as a full color image forming mode in which about equal amount of yellow, magenta, cyan and black toners are consumed.
For example, papers for business use consume black toner overwhelmingly. A specific color toner is sometimes consumed markedly in comparison to other colors, when a large amount of paper having a company logo or price list called “pop” used in super markets is printed. Consumption amount of color toners sometimes varies among toners since a specific color toner is frequently consumed according to the image forming mode.
The largely consumed color toner is supplied and charging characteristics of newly supplied toner must be arisen quickly to an available level for image forming.
The image forming unit becomes more compact, the quicker the charging characteristics of newly supplied toner must be arisen in an apparatus with four developing units having uniformed size. However it is difficult because the unit is made compact, and particularly, it is more difficult to raise charging characteristics of largely supplied within the developing unit when toner consumption amount varies among each color toners.
When a toner which is not prepared to have sufficient charging characteristics is used for image forming, such toner is not only form an image on a photoreceptor but it scattered to stain the interior of the image forming apparatus. In particular, it is problem to deteriorated environment of the apparatus when small size toner is employed since the small size toner is apt to scatter and leak outside of apparatus.
Patent Document No. 1: Japanese Patent L.O.P. 2001-209202
Patent Document No. 2: Japanese Patent L.O.P. 2001-201879